Sepsis or septic shock refers to a serious infection, localized or systemic, that is accompanied by systemic manifestations of inflammation. Sepsis due to bacteremia is often called septicemia. The more general term, systemic inflammatory response syndrome, recognizes that several severe conditions (e.g., infections, pancreatitis, burns, trauma) can trigger an acute inflammatory reaction, the systemic manifestations of which are associated with release into the bloodstream of a large number of endogenous mediators of inflammation.
It has been reported that sepsis is associated with unregulated apoptosis of cells of the immune system, particularly of lymphocytes (reviewed by Wasche et al., 2005). The critical ill patient/animal unregulated lymphocyte apoptosis in the thymus spleen, and gut-associated lymphoid tissue (GALT) may lead to immune suppression, leaving the patient/animal vulnerable to subsequent infections or unable to fight the existing sepsis, resulting in multiple organ failure (MOF). In addition, the inability of macrophages to clear dying lymphocytes/cells appropriately may allow the cells to progress to a state of secondary necrosis, producing localized bystander injury in the tissue.
One experimental model of sepsis is mice injected with increased doses of lipopolysaccharide (LPS), a component of the wall of Gram-negative bacteria. The model yields more than 85% mortality. LPS induces substantially high levels of TNF, IL-1, IL-6, and the chemokines KC and MIP-2. It has been reported that administration of LPS from Gram-negative bacteria leads to activation and massive apoptosis of T lymphocytes (Castro et al. 1998).
Another model of sepsis is cecal ligation and puncture (CLP). Mice subjected to CLP-induced peritonitis manifest an early hyperdinamic metabolic response and as sepsis progresses, hypodynamic phase and death.
Bommhardt et al. (2004) reported results on animal model of sepsis showing that a major defect in sepsis is impairment of adaptive immune system suggesting that strategies to prevent lymphocyte apoptosis represent a potential important new therapy in sepsis. The results in the report show that mice which overexpress the constitutively active serine/threonine kinase (Akt), which is a potent regulator of cell proliferation and cell survival and prevent cell death in a variety of settings, showed less apoptosis in lymphocytes and improved survival following induction of sepsis.
Organ damage and mortality associated with sepsis in mouse models was reported to be caused, at least in part, due to induction of apoptosis trough activation of the Fas-FasL signaling pathway (referred herein as the “Fas pathway”). Recently, it was reported that antiapoptotic treatments improve septic outcome (Wasche-Soldate et al. 2005). For example, injection of mice with small interfering RNA (siRNA) targeted against caspase-8 (to reduce caspase-8 in cell), a downstream caspase in the FAS pathway, was shown to attenuate the onset of morbidity and mortality in sepsis. Silencing of caspase-8 by siRNA suppressed apoptosis in the spleen and liver and conferred a significant increase in the survival of septic mice. The types of cells that are targeted by the siRNA treatment, treatment that enables the animal to survive in this model of sepsis, were not identified.
Caspases are known as intracellular cysteine proteases having a central role in apoptosis. Caspases are synthesized as inactive pro-enzyme precursors and are activated by proteolytic processing. The proteolytic activity of the caspases is required for the execution of cell death.
The release of activated caspases from apoptotic cells was reported to occur in pathological situations that are characterized to be associated with classical apoptotic death. The reports do not provide any clue as to the functional significance of this release. Speculations raised in them as to this functional significance were only about the possibility that the extracellular caspases have by themselves some prejudicial effects. For example, significant caspase cleaving activity was found in plasma of mice with fulminant CD95-triggered hepatitis, and in cerebrospinal fluid (CSF) from traumatic brain injury (TBI) patients (Harter et al. 2001 and Hentze et al. 2001). Harter (2001) shows the presence of active caspase-3 in CSF from patients with severe head injury. The report shows that active caspase-3 is released from apoptotic cells in the injured brain. The measured activity is the proteolytic activity (DEVDase activity) of caspase-3 in CSF collected from patients with TBI.
Hanze (2001) evaluated the stability of caspase-3 in extracellular environment in vitro and in vivo after intravenous injection in mice. The measured activity is the proteolytic activity of caspase-3 using the DEVD artificial substrate. It was found that the plasma DEVDase activity decreased rapidly (T½=15 min.), but was still detectable after over 60 min. It was shown that caspase-3 like activity was released from cell lines exposed to apoptotic stimulus and that release of caspase occurs in pathological situations that are well characterized to have major involvement of classical apoptotic death. For example, DEVDase activity was present in plasma of a mice hepatitis model involving apoptotic liver damage. Also, elevated DEVDase activity was found in liquor of patients with traumatic brain injury. It has been proposed by Hantze et al. that the caspase enzymatic activity might serve as a diagnostic marker for massive apoptotic organ damage. Hantze et al. (2001) speculates that apart from being indicators of damage, extracellular caspases might in fact have defined targets in pathological situations and further stated that elucidation of such interactions remains a challenge for the understanding of fulminant apoptotic organ damage.